Recently, the mode of cultivation resorting to the use of plug seedlings has disseminated in the fields of horticulture and afforestation. In this area fertilizers capable of furnishing fertilizing effect over periods ranging from one month to one year are demanded for the sake of pot seedlings and potted plants.
The conventional fast-releasing fertilizers using ammonium sulfate, urea, and ammonium phosphate are incapable of furnishing fertilizing effect over periods ranging from one month to one year. Some of the slow-releasing granular fertilizers having particle diameters in the range of from 2 to 10 mm are capable of furnishing fertilizing effect over a period of several months. They, however, have a particle size distribution. When such a fertilizer is applied to pots at a rate fixed by the number of grains, therefore, it tends to give rise to a difference in weight of fertilizer or a difference in efficacy of fertilizer among the pots under treatment. When this fertilizer is applied to the pots at a rate fixed by weight instead, the individual portion of the fertilizer to be supplied to each pot must be weighed out precisely one by one at the cost of time and labor.
As a slowly-releasing shaped fertilizer uniform in size, a molded briquette fertilizer which is produced by compression molding, using a roll press, a mixture of isobutylidene diurea, namely one of the reaction products of aldehyde and urea, and mineral heavy oil (as disclosed in Japan Tokkyo Kokoku 54-24,942, for example) has been known and has been widely used. This molded fertilizer excels in such properties as hardness, ability to retain shape in water, and strength to withstand the impact of fall. It shows, however, a fertilizing effect over a long period such as ranges from one to five years and, therefore, is unsuitable for uses which require the fertilizing effect over such a medium length of period as ranges from one month to one year. Besides, the roll press mentioned above poses a problem. The roll press permits easy production of molded fertilizers of 5 to 100 cm.sup.3 in volume. An attempt to produce lumps of fertilizer in a volume of less than 5 cm.sup.3 to obtain the fertilizing effect duration within a range of from one mouth to one year, however, inevitably requires a decrease in the size of the roll part of the roll press, with the result that the roll press will fail to generate necessary molding pressure and suffer from very poor molding capacity.
For the purpose of producing molded lumps of fertilizer with a volume of less than 5 cm.sup.3, an idea of adopting compression molding method by a tabletting machine to exert the necessary molding pressure may be conceived. When the compression molded fertilizer is produced by a tabletting machine, the obtained tablets are required to show fast shape retention and hardness and ensure high yield of production. So they must exhibit an exalted mold release property and be precluded from such adverse phenomena as capping (surface excoriation) and lamination (layer separation). The addition of an inorganic salt of stearic acid or talc has been known as means to improve the mold release property (Japan Tokkyo Kokai 59-190,287). The addition of an inorganic salt of stearic acid, however, is liable to induce the phenomenon of capping and lamination. A method which is capable of producing compression molded tablet of fertilizer excelling in moldability and yield of production has never been established. A method which attains the improvement of the moldability of fertilizer by the addition of an inorganic salt of lignin sulfonic acid has been known to the art (Japan Tokkyo Kokai 63-123,889 and Japan Tokkyo Kokoku 54-24,942). This method, however, poses a problem of poor mold release after the step of compression molding. Further, in the molded fertilizer produced by these methods, the fertilizer components are not uniformly distributed in the fertilizer composition, because the ingredients of fertilizer vary in grain size and other properties. Thus, the individual tablets of compression molded fertilizer reveal considerable inconstancy of distribution of fertilizer components and, therefore, are incapable of permitting accurate fertilization.